Tunable Photoanode-Photocathode-Catalyst Interface Systems for Efficient Solar Water Splitting

G. Charles Dismukes (PI), Martha Greenblatt & Eric GarfunkelRutgers UniversityJune 9, 2015

NSF-CBET/DOE-EERE jointProject ID# PD121

This presentation does not contain any proprietary, confidential, or otherwise restricted information

Department of Chemistry & Chemical Biology

35 x 35 nm

≈ 60°

ZnTPP Island

exposed Au

Fe0 Fe0

Fe2+Fe2+

Fe 2pCore Level

Replace costly low abundance noble metals used as catalysts in PECs & electrolyzersShift from corrosive acid to alkaline electrolytes that extend materials durability w/o efficiency lossPhotoanode materials for improved light to photochemical efficiency

•Project Start Date: Sept 1, 2014•Project End Date: Aug 31, 2017•Percent complete: 20%

Timeline Barriers addressed

Dr. Peter Khalifah (Stonybrook & BNL) perovskite received

Dr. Andrew Rappe (Upenn) Ni5P4 HER theory

In planning:• Dr. Yu Seung Kim (LANL) AEM• JCAP: testing

Unfunded Partners

Overview

•Total Project Budget: $749,996•Total Recipient Share:NSF disallows•Total Federal Share: $749,996•Total DOE Funds Spent*: $31,766

* As of 3/31/15

Budget

2

Objectives: Long-term: develop the knowledge base of materials chemistry needed to predict and

fabricate semiconductor/catalyst interfaces, both at the photoanode and photocathode, that attain or exceed the DOE benchmark STH efficiency (~10%)

Specific to Current Year: • Synthesize semiconductor light absorbers for longer wavelength absorption: selected phase-pure crystalline members of the perovskite oxynitride series ABO3-xNx, where A = alkaline earth or rare earth and B = Ti, V, Zr, Nb, or Ta

• Prepare thin films of OER catalyst cubic LiCoO2 both in direct contact with the photoanode and as a buried junction with intervening transparent conductor (TCO)

• Develop alkaline electrolytes compatible with both the OER and HER half-reactions

Relevance to DOE H2 & FC Program Materials Efficiency (Barrier AE) Photoanode materials for improved light to

photochemical efficiency – perovskite oxynitrides Auxiliary Materials (Barrier AI) Replace costly low abundance noble metals used as

catalysts in PECs & electrolyzers - thin films of OER catalysts cubic LiCoO2 Materials Durability- Shift from corrosive acid to alkaline electrolytes that extend

materials durability w/o efficiency loss – developed alkaline compatible HER catalyst

Relevance

3

• Using pulsed laser excitation deposit photoanode as thin film structure

•Deposit onto pn-silicon photocathode Ni5P4HER catalyst

• Investigate two electrolytes system: aqueous alkaline electrolyte solution (pH=14) and alkaline exchange membrane

•Device expected: Photoelectrochemical cell with STH efficiency of ~10%

A: alkaline earth/rare earth

B: Ti, V, Zr, Nb, Ta (d0)

O, N

AB(O,N)3, Optical Bandgap: 1.7 ~2.1 eV

bias (ΔE)

H2OO2

alkalineelectrolyte

AB(O,N)3

Cubic LiCoO2

H2O

H2h+

e-

hν

e-

h+

e-h+

anodeNi5P4-pn-silicon

cathode

CB

VB

hν

• Synthesize and investigate selected phase-pure members of perovskite oxynitride series AB(O,N)3 as long wavelength absorbers

• Characterize the optical bandgaps and photoinduced carrier lifetime of these materials in preparation of attaching OER catalyst

Light absorber: perovskite oxynitrides

Approach - Overview

Approach - OverviewIntegrated Tandem PEC with STH up to 10%

materials

Photoanodes(1.7eV-2.1eV)

Photocathode(above 0.8 eV)

interface

OER catalyst interface

HER catalyst interface

electrolytes

Perovskiteoxynitrides

pn-silicon junction Cubic LiCoO2 Ni5P4

1) Aqueous alkaline solution

2) Alkaline exchange membrane (AEM）

Project Focus: longer wavelength absorber for photon capture & conversion Electrosynthesis methods to create photoelectrode-interfaces to OER/HER catalysts alkaline electrolyte for trouble-free op.

Rutgers team approach to high solar-to-H2 efficiency

5

Approach – this year Photoanode materials with tunable optical absorbance

– perovskite oxynitrides ABO3-xNx

Method: convert layered perovskite oxides ABO3 to perovskiteoxynitrides by topological nitridization using NH3 treatment

Future efforts (FY 2015): Optimize the stability & bandgap by tuning the N content

(realized by doping the B site transition metal & controlling charge balance)

Layered perovskite, [110]-phases (AnBnO3n+2)

A cationB(O,N)6 octahedron

topologicalnitridation

NH3 flow

Merit: easy to adapt to thin film preparation

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Approach – this yearHow to selectively form a thin film of the cubic form of LiCoO2 OER catalyst onto a conducting oxide?Obstacles: avoid forming inactive layered LiCoO2 polymorph and unstable Co3O4 spinel.Achieve uniform surface coverage with minimum defects and thickness Achieve low impedance (conducting) interface

Future Effort (FY 2015): Apply above methods to fabricate these interfaces:OER/ photoanode perovskite absorberHER/photocathode pn-Silicon absorber

Achieve minimum optical defects & retain the above interfacial gains

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Two phase-pure oxynitridesLaNbON2 and SrNbO2N have been synthesized by using the topological nitridation of the oxide precursors.

Accomplishments: synthesis of ANb(O,N)3 A = La & Sr

LaNbO4 LaNbON2Ammonia treatment at high temperature

Sr2Nb2O7 SrNbO2N

Structural validation by powder XRD

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Results: Both LaNbON2 and SrNbO2N showed

significant absorbance in the visible light region.

However, the optical bandgap is hard to determine from the UV-Vis spectra. The tail may be due to the partial reduction of Nb from 5+ to 4+. Further efforts will be made to modify the nitridization conditions.

Achievement to the goal: According to the design of our

tandem PEC configuration, the photoanode requires a semi-conducting material with a bandgap in the range of 1.7eV ~ 2.1eV.

These oxynitrides are good candidates for this application, although further efforts are still necessary to optimize the optical properties.

Accomplishments: Optical properties of ANb(O,N)3 A = La & Sr

La Sr

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Accomplishments: Electrosynthesis of OER thin films

Novel electrochemical synthesis of cubic spinel LixCoO2 (0≤x≤1) film was achieved. Appropriate thickness confirmed by Scanning Electron Microscopy (SEM)Surface specific chemical composition of Co, O & Li determined by Rutherford Backscattering Spectrometry (RBS), & Nuclear Reaction Analysis (NRA)OER film performance confirmed by a) i-V polarization, b) O2 production confirmed by GC, and (c) ohmicresistivity determined by electrochemical Impedance spectroscopy.

Bare TCO electrode LixCoO2/TCO electrodeElectrochemical

deposition

-0.2 0.0 0.2 0.4

0

10

20

30

40

50

Cur

rent

den

sity

(mA

cm-2)

Overpotential (V)

LixCoO2/FTO FTO

10 mV_s in 1 M NaOH

SEM of LixCoO2/FTO film before (L) & after (R) 14 h electrolysis

15 18 21 24

0

-3

-6

-9

Z' (Ω

)

Z (Ω)

EIS at OER of LixCoO2

DC potential at 0.446 V in 1 M NaOHRs =15 Ω, Rct = 8 Ω

RBS NRA

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La2Ti2O7 single crystal* perovskite to be used for developing pulsed laser desorption (pld) methods for OER film formationOER laser target for pld transfer was prepared from cubic LiCoO2materialHER laser target for pld transfer was prepared from Ni5P4

Access & training to Rutgers PLD system identified

* provided by Prof. Peter Khalifah

Accomplishments: Towards Perovskite absorber-OER interface

La2Ti2O7 single crystals

LiCoO2 pellet for pld target

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No reviews. This project began on Sept 1, 2014.

Accomplishments: Responses to previous year’s reviewers’ comments

12

Collaborations-Unfunded Partners – this year: Stony Brook University

Prof. Peter Khalifah – La2Ti2O7 single crystal sample received. For use in developing PLD methods for transfer of OER material to perovskiteabsorber

• UPenn Materials Science and Pennergy CenterProf. Andrew Rappe – computational studies initiated of nickel phosphides and their interfaces for applications to interfacial charge transfer and HER catalysis

• Proton OnSiteDr. Katherine Ayers – will test OER activity of LiCoO2 in alkaline membrane electrolyzer stack

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Remaining Challenges and Barriers• Evaluation of the stability of

perovskite oxynitrides-electrolyte junctions under illumination will be key challenges to evaluate in the coming year (no effort thus far)

• Formation of charge transfer interfaces between photocathode and existing HER Ni5P4 catalyst, and between photoanode and existing OER LiCoO2 will be key challenges to evaluate in the coming year (no effort thus far)

to vacuum pump

targetpulse laser beam

laserplume

TCOs

TCOs

TCOsLiCoO2

AB(O,N)3

LiCoO2AB(O,N)3

substrate

PLD photoanode

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Proposed Future Work

0 0.5 1 1.5 2 2.5 3

SC opt tuning

SC/OER Interface

SC/HER Interface

AEM Stack

Timeline of Activities

Design cell electrical contact-thin films

electrolyte contact-ionomercorrosion pasivation

BG tuning TCO contact morphologyCB/VB E0 tuning

corrosion pasivation

Ionomer /cat AEM select

System imped

A timeline of expected activities during the 3 year grant period broken down in five activities:

1) Photoanode optical tuning2) Photoanode/OER catalyst interface 3) Photocathode/HER catalyst interface4) Alkaline exchange membrane/PEC

stack integration5) System testing

In 2015-16 we shall:•Optically characterize photoanodeabsorbers: LaNbON2 and SrNbO2N•HER Ni5P4 stability/activity with AEM*•OER LiCoO2 stability/activity with AEM*•Create these interfaces by PLD

oHER Ni5P4 /pn-SiliconoOER LiCoO2 /perovskite

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Technology Transfer Activities

Evaluation of LiCoO2 as potential replacement for IrO2anode in alkaline electrolyzer configuration will be conducted by Proton OnSite (Dr. Katherine Ayers)

Successful synthesis of phase pure forms of two perovskite oxynitrides: LaNbON2 and SrNbO2N, has been achieved

Optical characterization of these materials as long wavelength absorbers is in progress

Novel electro-synthesis method developed for formation of thin films of cubic LiCoO2 OER catalyst

Successful testing of OER LiCoO2 and HER Ni5P4 catalysts in alkaline electrolyte, pH 14 NaOH

PLD targets made for OER and HER materials and PLD in progress

Archtypical perovskite single crystal acquired for thin film deposition

Summary

G.C. Dismukes, E. Garfunkel, M. Greenblatt. Poster presented at the AMR Hydrogen program annual review (June 9 2015). Tunable Photoanode-Photocathode-Catalyst Interface Systems for Efficient Solar Water Splitting

G. C. Dismukes (Rutgers University), A. Laursen, B. Liu, K. Patraju, and M. Greenblatt (Rutgers). Oral presentation to the Electrochemical Society, Chicago, May 28 "(Invited) Renewable Hydrogen Evolution on Nickel Phosphide Electrocatalysts: A Comparative Study of Efficiency and Tolerance to Corrosion"

G. Gardner, P. Smith, C. Kaplan, J. F. Al-Sharab, Y. B. Go, M. Greenblatt, and G. C. Dismukes (Rutgers University). Oral presentation to the Electrochemical Society, Chicago, May 28. "Understanding the Influence of Structure on Activity and Stability in the Catalysis of the Oxygen Evolution Reaction (OER) Using Crystalline Oxides As a Platform"

Bin Liu, E. Garfunkel, M Greenblatt, and G. C. Dismukes. Poster presented to the Metropolitan New York Catalysis Society annual meeting March 18 (award poster) “Perovskite-related oxynitrides as photoanodes in photoelectrochemical cells”

Bin Liu, E. Garfunkel, M Greenblatt, and G. C. Dismukes. Poster presented to the 29th Annual Symposium of the Laboratory for Surface Modification April 2. “Perovskite-related oxynitrides as photoanodes in photoelectrochemical cells”

Publications & Presentations (this year)